In the field of optics, the use of holograms as filters has been recognized as an effective way to protect eyes, instruments, and/or detectors from damaging narrow wavelength band light rays, such as light being encountered from a laser beam.
A typical hologram is constructed from a layer of photoreactive gelatin on a substrate exposed to laser beams. The laser beams set up an interference pattern which is recorded in the gelatin as a sinusoidal modulation in the index of refraction of the gelatin. This pattern of modulations defines a hologram. By suitable control of the lasers used to generate the hologram, the degree of reflectivity and transmissivity of the hologram can be selectively determined and a cone of total reflection for a given wavelength can be provided.
A hologram intended for use in a light protection device is designed to diffract and redirect harmful rays of light out of a user's field of view. It is important that such a hologram maintain over time the same degree of reflectivity at the peak wavelength as that originally recorded so that the cone of total reflection remains essentially the same for effectively diffracting the anticipated harmful light rays.
However, moisture coming in contact with a holographic gelatin substrate can change the degree of reflectivity at the peak wavelength and produce a wavelength shift or drift. Materials have been found that protect the hologram from moisture for up to a year at high efficiency. However, protective coatings are often fragile and a system for verifying their effectiveness is desirable. Glass has been shown to be effective and can be used to provide a standard for comparison, but it is generally undesirable for covering the entire hologram due to weight and safety considerations.
Previous efforts have concentrated on eliminating the moisture problem, rather than monitoring it. A spectrophotometer can measure actual transmission of the hologram, but it lacks portability. Special devices with bandpass filters of the proper wavelength also can be useful for monitoring purposes, but they would require some expertise to use, as well as periodic calibrations and upkeep. Other devices have been postulated using either a narrow bandpass filter of the proper shape and wavelength, or two narrow filters at the 50% points applied to the surface of the hologram substrate. In the first case, an excess of visible light would signal that the wavelength had shifted too much, and in the second case, more light through one filter than through the other would indicate the presence of moisture damage. In both cases, user judgment comes into play, and although standards could be built into the device, it is not entirely satisfactory. Furthermore, producing the proper narrow band filters could cause production problems.
A need exists for a convenient method of visually monitoring the condition of the hologram using a device built right into the light protection device.